1. Field of the Invention
This invention relates to gas turbine engines and more particularly to engines having a variable geometry compression section.
2. Description of the Prior Art
In a gas turbine engine of the type referred to above, working medium gases are compressed by a first series of rotor mounted blades in a compression section and are flowed axially downstream to a combustion section. Fuel is combined with the compressed gases and burned in a combustion section to add thermal energy to the flowing medium. Downstream of the combustion section the medium gases are flowed across a second series of rotor mounted blades which are located in the turbine section. The second series of blades extract sufficient energy from the flowing gases to drive the blades of the compression section.
In an axial flow engine the blades of the compression section are arranged in rows which extend radially outward from the rotor into the working medium flow path. A compressor case surrounds the blades and the rotor. Rows of compressor vanes are circumferentially disposed across the flow path radially inward of the case between each pair of adjacent blade rows. Each row of vanes directs the medium gases discharging from the immediately upstream row of blades to a preferred angle of entry into the immediately downstream row of blades. The preferred angle of entry into each row of blades varies according to the speed of rotation of the rotor and the velocity vector of the medium gases approaching the blades.
Modern compressors having optimized flow characteristics contain vanes within the compressor section which are rotatably mounted with respect to the compressor case for conforming the flow thereacross to a preferred angle of entry into the downstream blades irrespective of the rotor speed or the velocity vector of the gases at any particular operating condition. Compression sections of this type are well known within the art and are termed "variable geometry compressors". Variable geometry compressors are more fully discussed in U.S. Pat. No. 2,805,818 to Ferri entitled "Stator of Axial Flow Compressor with Supersonic Velocity at Entrance", U.S. Pat. No. 2,999,630 to Warren et al entitled "Compressor", and U.S. Pat. No. 3,873,230 to Norris et al. entitled "Stator Vane Actuating Mechanism".
Conventionally, the flow discharging from the last row of compressor blades has a tangential velocity within the flow path which is in the direction of rotation of the rotor. A row of vanes is positioned downstream of the last row of blades to redirect the medium gases flowing thereacross to an essentially axial direction as the flow approaches the combustion section. Apertures at the upstream end of a combustion chamber within the combustion section are fixedly oriented to accept flow from the axial direction in the attainment of optimum combustion characteristics. Accordingly, the last row of vanes upstream of the combustion section is fixed relative to the flow path so as to discharge the flow axially into the combustion section regardless of the engine operating conditions.
Continuing efforts are underway to effect aerodynamic improvements in the flow of air from the compression section to the combustion section of a gas turbine engine while maintaining characteristics which are consonant with optimum operation in the combustion section.